This invention relates to a start-up method for ammonia plants which produce a hydrogen-containing synthesis gas from hydrocarbon feed stock.
Almost all hydrogen and hydrogen-containing synthesis gases are produced by passing a heated gaseous stream of hydrocarbons and steam over nickel-containing catalysts. A major use of hydrogen is thus produced for the production of ammonia which occurs in the same plant which produced the synthesis gas. In the United States and other areas of the world with an abundant supply of natural gas, the preferred hydrocarbon feed is natural gas. In areas where natural gas is not readily available, the preferred feed stock is light naptha.
Plants that produce hydrogen and hydrogen-containing synthesis gases by the steam-reforming of hydrocarbons are subject to frequent start-ups and shutdowns. For example, a recent survey of ammonia plant shutdowns reported that the worldwide average for ammonia plants is 9 shutdowns per year. Williams, G. P. and Hoehing, W. W., "Causes of Ammonia Plant Shutdowns" Survey IV. AIChE Ammonia Safety Symposium, (November, 1982). Of these, 5 are due to major equipment failures, which probably necessitated a complete shutdown and re-start from ambient temperature conditions.
Such start-ups involve gradually raising the temperature of the catalyst beds in the various stages (primary and secondary reformers and carbon monoxide conversion stages) to their operating temperatures while passing a stream of hydrocarbon feed stock therethrough. Because unconverted or partially converted feedstock from one stage can damage the catalyst in the bed of a stage which follows, the unconverted or partially converted feed stock stream is vented and flared until all stages become operational, first usually after the high temperature shift stage, then after the CO.sub.2 scrubbing stage and finally after the methanation stage.
Prior to the mid-70's, this wasting of natural gas as part of a start-up procedure was not a serious economic factor in the plant operation. However, in the early 1980's, rapid and dramatic increases in the cost of energy, especially natural gas and naptha, caused the cost of start-up of such plants to become excessively high. For example, Hays Mayo, in his paper presented at the August, 1985 AIChE Ammonia Symposium, entitled "Low Energy Accelerated Start-up of Ammonia Units", reported that the average cost of a start-up of a typical 1,000 t.p.d. natural gas-based ammonia plant was 30-35 billion BTU's and required 30 hours time. Therefore, the natural gas cost of each start-up was $75,000, assuming a price of $2.50 per million BTU, or an average annual expenditure of $375,000 per plant for natural gas alone. For naptha units, the cost would be approximately double the above figures due to the higher cost of naptha.
For these reasons, in 1983 Mayo developed a "Low Energy Accelerated Start-Up Process For Ammonia Plants" featuring more rapid heating rates for the primary reformer and synthesis sections of the plant. Mayo estimates his process reduces the energy costs for the average 1,000 t.p.d. ammonia plant based on natural gas from 30 to 35 down to 10-12 billion BTU and reduces the start-up time by 18 hours.
For naptha based plants, the savings using the Mayo procedure are estimated at 15 billion BTU's or $90,000 per start-up.
Others have considered reducing the cost of ammonia plant start-ups. Madhaven, S. and Kearney, D. J., in a report entitled "Reduced Energy Ammonia Plant Restart", presented at the Kellogg Ammonia Club Meeting of November, 1984, covered pertinent aspects of this subject and confirmed many of the recommendations of Mayo. As a result of these savings, the Mayo process has been quickly and widely adopted by the ammonia industry. Features of the Mayo process include raising the heating rates in the plant to 300.degree. F. per hour and to reduce the process gas usage from about 50% of design to about 25% of design during the heat-up process.
It can be seen however, that flared feedstock hydrocarbons still represents a major start-up expense for such plants ($150,000/yr./plant).
This invention, in one of its embodiments, will result in an estimated additional savings beyond those achieved by the Mayo procedures of 6-9 billion BTU's per start-up of a natural gas--based 1,000 t.p.d. NH.sub.3 plant and ordinarily even more than that for naptha-based plants. In addition, this invention will provide other advantages to producers of hydrogen and hydrogen-containing synthesis gases.
For example, in the Mayo procedure, natural gas is introduced to the primary reformer of an ammonia plant at 1400.degree. F. The possibility exists that if the natural gas is added too rapidly or at somewhat lower temperatures, coking or carbon formation may form on the catalyst. This is especially true if some tubes are cooler than the bulk temperature of 1400.degree. F., due to non-uniformity of firing the reformer. This carbon formation could result in hot tubes in the furnace curtailing production rates and, under the worst conditions, could cause a shutdown and replacement of the catalyst. Thus, the potential $50-60,000 savings in energy cost could become a potential $300,000 cost for catalyst replacement plus the loss of 3-5 days of production, if the plant experiences coking and requires a catalyst replacement. This invention eliminates this possibility.
Another advantage of this invention is the elimination of a possibility of feeding carbon oxides, which are poisons for the ammonia synthesis catalyst, into the synthesis section of the plant as a result of maloperation of the methanator.
Another advantage of this invention is the elimination of the possibility of nickel carbonyl formation and the release of nickel carbonyl to the atmosphere during the early stages of heat-up of the methanation catalyst. The carbonyl formation requires carbon oxides in the gas and the start-up media of this invention contains no carbon oxides.
Another advantage of this invention is the elimination of the time delay in lining out the CO.sub.2 removal system before introduction of the gas to the methanator. This procedure is necessary in previously used start-up procedures because, if there is a high CO.sub.2 content in the feed to the methanator, excessive heat of reaction is possible from the methanation of carbon oxides which could lead to damage of the methanation catalyst and, in the worst case, damage to the methanator vessel.
Still another advantage of this invention is that no time delay is required between the start of the heat-up process in the "front-end" or synthesis gas preparation section of the plant and the start of the heat-up process in the synthesis section of the plant. In the Mayo process, this delay amounts to approximately 3 hours or 3 billion BTU's.
Yet another advantage of this invention is that there is little or no loss of the start-up media other than leaks in the system. In the Mayo process, the synthesis gas produced by the reforming process is flared or vented for 6-8 hours providing a loss of 6-8 billion BTU's in energy costs. This invention requires no flaring or venting and the start-up media (except for leaks) is completely recoverable.
Yet another advantage of this invention is the elimination of the need to operate the air compressor at the inlet of the secondary reformer during the start-up process, thus saving additional energy.